Method of removing impurities from a compressed gas



D. R. FRIANT May 28, 1963 METHOD OF REMOVING IMFURITIES FROM ACOMPRESSED GAS Filed Aug. 11, 1960 INVENTOR DAV/D Ff? /VT HIS JerseyFiled Aug. 11, 1960, Ser. No. 48,874 6 Claims. (Cl. 62-85) Thisinvention relates to a method of removing impurities from a compressedgas and is primarily concerned with atnt a method of removing watervapor vand oil vapor from compressed air.

In the past it has been the practice to use one or more compressionstages in association with desiccant cartridges. First the gas goesthrough the compression stage or tages and then it goe through thedesciccant cartridges which remove water vapor or other impurities. Thedesiccant cartridges have to be replaced every so often and it takestime and eiort to replace them and the attendant may forget to do itwhich could cause liquid water to get into the machine -in which it isto be used. There is no easy and reliable way to measure dew point inthe desiccant cartridge method. A desiccant cartridge is a chambercontaining a 'filter and a desiccant such `as silica gel for removingwater vapor or other Iimpurities from a gas.

In the past it has also been the practice to reactivate the desiccantperiodically by any of several means. In this desiccant method it isdicult to measure the dryness of the `air being delivered and it costsat least $1000 -for the regenerative type regardless of size.

In the past it has further been the practice to move air through a heatexchanger which is cooled by the outgoing air. Then the air ows to acondenser evaporator. Here the final cooling of the air takes place.Moisture and oil vapors are condensed and drop into the condensatecollection chamber. The air is then passed through a lter cartridgeelement which collects foreign matter. The cool, dry `air then re-entersthe heat exchanger where it is reheated by the incoming hot air beforeentering the lines to the receiver or storage tank. The disadvantages ofthis method are that a separate refrigerant compressor and tanks and anexpansion lorifice are all needed.

The method of the present invention is comprised of a plurality ofcompression stages and in each stage the pressure is increased and thetemperature is increased over what it was in the preceding compressionstage. A cooling means is used to cool the gas after each compressionstage resulting in a decrease in the temperature of the gas and in amajority of `the locations the cooling means increases the eiciency ofcompression by approximating an isothermal process. An expansion stageis provided which decreases the pressure and decreases the temperature.The method of the present invention provides extremely low temperatures.One advantage of the present invention is that the temperature can bemeasured to find out how dry the `air is. The Iinvention has many otheradvantages which are set fort-h later in this specification.

An object of the invention is to provide an improved method ot removingimpurities from a compressed gas.

Another object of the invention is to provide a method of removingimpurities from a compressed gas which does not require a separaterefrigerant.

Another object of the invention is to provide a method of removingimpurities from a compressed gas wherein measuring devices andcontrolling devices and warning devices can easily be used.

A further object of the invention is to provide a method A still furtherobject of the invention is to provide a method of removing water vaporfrom compressed air wherein no provisions are necessary for trappingcondensed water beyond the third compression stage.

The foregoing and other objects of the invention will be apparent fromthe arrangement shown in the accompanying drawing wherein:

The FIGURE is a schematic drawing of .a method of removing water vaporand oil vapor trom compressed air.

In the present invention there are ve stages of compression. These vecompression stages can all take place in one compressor and preferably asingle compressor is ued for all tive stages. However, a plurality ofcompressors can be used for the iive stages if desired. The compressorcan be of the piston and cylinder type, the bladed type, the vane type,or the lobed type. In the present in-\ vention an expander is requiredto accomplish the expansion stage. This expander can be a piston andcylinder, a turbine, a vane motor, or a gear motor. Oil -is ordinarilyused to lubricate the moving parts of the compressor and:4

it is ordinarily desirable to remove the resulting oil vapor. from theair.

In the drawings, air from the atmosphere enters the rst compressionstage 10 at point A. In the rst compression stage 10 the pressure of theair is increased and the temperature of the air is increased. The airthen passes into an intercooler 11 Where its temperature is decreasedand some of the water vapor is condensed into water land some of the oilvapor is condensed into liquid oil and the decrease in temperature alsoprevents heat damage to the oil and the parts. The intercooler 11increases the eciency of compression by approximating an isothermalprocess. The Water vapor that condenses into water in intercooler 11 andthe -oil vapor that condenses -into liquid oil ilow into a condensatetrap 12. At the lower end of the condensate trap 12 a manually operatedor automatically operated Valve 13 is provided whereby the condensedwater can be drained into a drain trough 14. The air then flows into thesecond compression stage 15 where its pressure is increased and itstemperature is increased and the air then flows through an intercooler16 where its temperature is decreased and some of Ithe Water vapor andoil vapor is condensed into water and oil respectively -and thiscondensed water and condensed oil flow into a condenser 17 and out thebottom of the condenser into a condensate ytrap 18. At the lower end ofcondensate trap 18 la manually operated or automatically operated valve19 is located and upon operation of this valve the condensed water andcondensed oil are drained into a drain trough 20. It is to be noted thatthe drain troughs 14 and 20 are not necessary as the condensed oil andthe condensed water can be drained on the ground. The air then entersthe condenser 17 and il'ows around a cooling coil 21 where itstemperature is decreased Iand this cooling oil causes some of the watervapor to condense into water and to drain into the condensate trap 18.The pressure of the air in the area of cooling coil 21 staysapproximately constant. The upper part of coil 21 may have frost or iceon it. The air then ows around a cooling coil 22 where its temperatureis reduced. The pressure of the air in the area of cooling coil 22 staysapproximately constant. The cooling coil 22 may have frost or ice uponit. The condenser 17 can be relatively small because there is less waterto be condensed than at previous points in the process and because theair has a low specific volume and hence stays in the condenser for arelatively long time. The condenser can be constructed of relativelythin material because the pressure is not excessive. The air in .thecondenser 17 then ows into a conduit 23 and from this conduit it ilowsthrough the interior of the cooling coil 21 where it absorbs heat fromthe 3- air passing around the outside of the coolingroil. The air thenows through a conduit 24 into an expander 25 Where its pressure isdecreased and its temperature is decreased. The Vmain purpose oftheexpander 25 is to decrease the temperature'.V Theexpander 25 can'berelatively 'small because the expansion'takes place at ya highY pressureand therefore ItheV air occupies relatively little space.V However, thepressure is not so high that strength and volumetric eiciency areserious problems. YAt point G all of the water that is going to condenseout has condensed out so that there will be no Water vapor hetween H andJ that will condense out even -though there is a minute amount of watervapor still left in the air. Thus condensation in the expander 25 isprevented. The air then iiows into a conduit 26 and then through theinterior of the cooling coil 22 where it absorbs heat from the airpassing around the cooling coil. The vair then enters the thirdcompression stage 27 where its pressure is increased and its temperatureis increased. The air then flows through an -intercooler 2S whichreduces the temperature of the -air and some of the oil vapor iscondensed into liquid oil. The intercooler 28 increases the eiiciencyofcompression by approximating an isothermal process. The air thenenters the fourth compression stage 29 where its pressure is increasedand its temperature is increased. The air then enters the intercooler 30where its temperature is decreased and some of the oil vapor iscondensed into liquid oil. The intercooler 30increases the eiiciency ofcompressionby approximately an isothermal process. The air then entersthe fth compression stagef31 where its pressure is increased and itstemperaturefis increased. The airthen flows through the aftercooler 32Where 4its temperature is decreased. This dry compressed a-ir is thenallowed Vto ow into la storage tank Where it is ready to be used.vIt-should be noted that the aftercooler 32 may be omitted and also thefifth compression stage 31 may be omitted and also the fourth and ii'fthcompression stages 29 and 31 may be omitted and also the third, fourthand lifth compression stagesV 27, 29, and 31 may be omitted.

The table below shows Vthe pressures and temperatures and pounds ofwater per pound of dry air at locations A, B, C, D, E, F, G, YH, I, K.L, M, N,'P, Q, and R. The figures listed lin this table'apply to anexample in which -ai-r vsaturated with Watervapor -at 70 F. and 14.7pounds per vsquare inch absolute is dried and compressed to 6090 poundsper square inch gauge. i

YFrom the table it will be peraturesare attained.

Theadvantages of applicants method are as lfollows: There l'is rnodesiccant required vand no replacement of cartridges. A separatelrefrigerant such as Freon -is not needed for the coils 21 and 22. YSincea separate refrigeraritis Vnot needed a refrigerant compressor and tanksand an expansion orifice are not needed and whichwould all'v be neededwhen a separate refrigerant is used. It Iis easy to put controls orwarning devices on applicants ap- `seen that extremely low temparatus,that is, the temperature at Gl can be measured to find out how dry theair is. The condensate trapping problem beyond the third compressi-onstage 27 is not as great as it would be if the expander 25 `andcondenser 17 were not used. No provisions are necessary for trappingcondensed water beyond the -third compression stage 27.

Any condensate beyond the third compression stage will be that of oilVapor into liquid oil. This oil is for lubricating the moving parts ofthe compressor. Although'oil carried over from one stage to Ithesucceeding stage does not provide the only lubrication for thesucceeding stage, the small amount of oil which condenses in the highpressure coolers assists in lubricating the moving parts of thesucceeding stage particularly the valves if the compressor is one of thepiston and cylinder type. In the third compression stage 27 since thetemperature is very low a higher compression pressure ratio can be-attained than in any other compression stage without damaging the partsof the compressor and without damaging .the lubricant which is oil. Ifoil gets too hot it will turn into a solid or gummy carbonaceousmaterial. 'Ihis carbonaceous material will deposit on the -valves andcause faulty operation of the valves. In the plain desiccant andVregenerative desiccant systems priority valves were needed.

A priority valve isgone which prevents air from flowing throngh thedesiccant cartridges unless the pressure exceeds some minimum value. Inapplicantsmethod no priority valvesVV are` needed to protect thedesiccant cartridges'since they Iare not u'sed in this apparatus.

Dry compressed air is used to start jet engines and if a slug of Watergets into the lblades of a jet engine it is likely to cause seriousdamage or even complete destruction of the engine. Dry compressed vairisalso used in instruments operated pneumatically.

This could also be a nitrogen compressor in which case one impurity iswater and in which pure nitrogen is desired. This could also be amethane compressor in which case one impurity may be butane and in whichcase pure methane is' desired.

Whatis claimed is:

1.. Armeth`od of removing kimpurities from a compressed gas comprisingthe following steps in the order named, of:

() alternately compressing and cooling the gas a plu-, rality oftimes toprogressively increase its pressure each time the-gas is compressed andto alternately increase and decrease its temperature at each increasedpressure for causing condensation of some of the 4impurities that drainoi each time temperature is decreased,

(b) passing' the gas through locations to decrease the temperature intwo steps at approximately constant pressure causing radditionalimpuritiesto condense and drain off,

(c) moving the gas through a location where it absorbs heat,

(d) expanding the gas to decrease its pressure and temperature,

(e) moving the gas through a location ,where 4it ab sorbs heat, (f)compressing the gas at least one time to increase its pressure andtemperature, and I (g) cooling the gas at its increased pressureso thetemperature of the compressed gas substantially corresponds to thetemperature of thejimpure gas prior to processing.

2. The method of removing impurities from a compressed gas comprisingthe following steps in the order named, of:

` (a) alternately compressing and cooling the gas a plurality of timesto progressively increase its pressure each time the gas is compressedandto alternately increase and decrease its temperature atreachincreased pressure for causing condensation of some of the impuritiesthat drain ofi each time temperature is decreased,

(b) cooling the gas in steps at substantially constant pressure causingadditional impurities to condense and drain ott by passing the gasthrough two heat exchange means in series with one another,

(c) heating the gas by passing it back through the rst of the two heatexchange means in series to prevent condensation when expanded,

(d) expanding the gas to decrease its pressure and temperature,

(e) heating the gas by passing it back through the second of the ltwoheat exchange means in series,

(j) compressing the gas at least one time to increase its pressure andtemperature, and

(g) cooling the gas at its increased pressure so the temperature of thecompressed gas substantially corresponds to the temperature of theimpure gas prior to processing.

3. A method of removing impurities from a compressed gas comprising thefollowing steps in the order named, of:

(a) alternately compressing and cooling the gas a plurality of times toprogressively increase its pressure each time the gas is compressed andto alternately increase and decrease its temperature at each increasedpressure for causing condensation of some of the impurities that drainoi each time temperature is decreased,

(b) passing the gas through locations to decrease the temperature in twosteps at approximately constant pressure causing additional impuritiesto condense and drain oi,

(c) moving the gas through a location where it absorbs heat,

(d) expanding the gas to decrease its pressure and temperature,

(e) moving the gas through a location where it absorbs heat,

(f) compressing the gas a plurality of times to increase its pressureand temperature each time the gas is compressed, and

(g) cooling the gas after each time it is compressed to decrease itstemperature at each increased pressure so the temperature remains withina constant range as the pressure progressively increases.

4. The method of removing impurities from a compressed gas comprisingthe following steps in the order named, of:

(a) alternately compressing and cooling the gas a plurality of times toprogressively increase its pressure each time the gas is compressed andto alternately increase and decrease its temperature at each increasedpressure for causing condensation of some of the impurities that drainoi each time temperature is decreased,

(b) cooling the gas in steps at substantially constant pressure causingadditional impurities to condense and drain oi by passing the gasthrough two heat exchange means in series with one another,

(c) heating the gas by passing it back through the iirst of the two heatexchange means in series to prevent condensation when expanded,

(d) expanding the gas to decrease its pressure and temperature,

(e) heating the gas by passing it back through the second of the twoheat exchange means in series, (f) compressing the gas a plurality oftimes to increase its pressure and temperature each time the gas iscompressed, and

(g) cooling the gas after each time it is compressed to decrease itstemperature at each increased pressure so the temperature remains withina constant range as the pressure progressively increases.

5. A method of removing impurities from -a compressed gas comprising thefollowing steps in the order named, of:

(a) alternately compressing and cooling the gas a plurality of times toprogressively increase its pressure each time the gas is compresse-d andto alternately increase and decrease its temperature at each increasedpressure for causing condensation of some of the impurities that drainoi each time temperature -is decreased,

(b) passing the gas through locations to decrease the lemperature in twosteps at approximately constant pressure causing additional impuritiesto condense and drain oit,

(c) moving the gas through a location where it absorbs heat,

(d) expanding the gas to decrease its pressure and temperature,

(e) moving the gas through ya location Where it absorbs heat,

(f) compressing the gas a plurality of times to increase its pressureand temperature each time the gas is compressed, and

(g) cooling the gas rafter each time it is compressed to decrease itstemperature at each increased pressure so the temperature each time thegas is cooled substantially corresponds to the temperature of the impuregas prior to processing as the pressure progressively increases.

6. The method `of removing impurities from a compressed gas comprisingthe following steps in the order named, of:

(a) alternately compressing and cooling the gas a plurality of times toprogressively increase its pressure each time the gas is compressed `andto yalternately increase and decrease its temperature at each ncreasedpressure for causing condensation of some of the impurities that drain`off each time temperature is decreased,

(b) cooling the gas in steps at substantially constant pressure causingadditional impurities to condense and drain `oi by passing the gasthrough two heat exchange means in series with one another,

(c) heating the gas by passing it back through the first of the two heatexchange means in series to prevent condensation when expanded,

(d) expanding the gas to decrease its pressure and temperature,

(e) heating the gas by passing it back through the second of :the twoheat exchange means in series,

(f) compressing the gas a plurality of times to increase its pressureand temperature each time the gas is compressed, and

(g) cooling the gas after each time it is compressed to decrease itstemperature at each increased pressure so the temperature each time thegas is cooled substantially corresponds to the temperature of the impuregas prior to processing -as the pressure progressively increases.

References Cited in the le of this patent UNITED STATES PATENTS1,264,399 Jones Apr. 30, 1918 1,881,116 Bottoms Oct. 4, 1932 2,077,315Ewing et al Apr. 13, 1937 2,134,699 Brewster Nov. 1, 1938 2,267,761Steward Dec. 30, 1941 2,818,454 Wilson Dec. 31, 1957 2,966,036 StowensDec. 27, 1960 FOREIGN PATENTS 909,568 Germany Apr. 22, 1954 1,233,248France May 2, 1960

1. A METHOD OF REMOVING IMPURITIES FROM A COMPRESSED GAS COMPRISING THEFOLLOWING STEPS IN THE ORDER NAMED, OF: (A) ALTERNATELY COMPRESSING ANDCOOLING THE GAS A PLURALITY OF TIMES TO PROGRESSIVELY INCREASE ITSPRESSURE EACH TIME THE GAS IS COMPRESSED AND TO ALTERNATELY INCREASE ANDDECREASE ITS TEMPERATURE AT EACH INCREASED PRESSURE FOR CAUSINGCONDENSATION OF SOME OF THE IMPURITIES THAT DRAIN OFF EACH TIMETEMPERATURE IS DECREASED,